The long-term goal of the proposal research is to make significant strides in understanding how the central auditory system encodes complex sound features. To achieve this goal, state-of-the-art neurophysiological and neuroanatomic techniques will be employed to characterize pathways that preferentially encode important dynamic features of complex sounds. Rapid changes in sound amplitude and frequency will be studied since they are essential components of many human and animal communication sounds, and in some cases mediate pitch perception and sound localization. The specific aims of the present proposal are to rigorously investigate the responses of single cells in the ventral cochlear nuclear to AM and FM signals in quiet and background noise. During single-cell mapping experiments, HRP will be ejected from the recording electrode extracellularly to mark recording site locations and trace their connections with higher centers. Single cells will be grouped into different populations based on responses to simple sounds, anatomical location, and extent of excitatory and inhibitory response areas. These population responses will be analyzed qualitatively and quantitatively, and to gain a better understanding of how cochlear nucleus cells process inputs they receive from the auditory nerve, responses of different cochlear nucleus populations will be compared to responses of auditory-nerve fibers obtained in the same species under identical recording, stimulation and anesthetic conditions. Lastly, to maximize implications for the mammalian auditory system, and in particular for the human auditory system in which single-cell studies cannot be done, experiments will be performed on an animal species that has the perceptual capability of discriminating complex communication sounds such as speech, and has an audibility curve similar to that of man. The results of the proposed research will be essential for clinician scientists developing and testing cochlear implants and the newer cochlear nucleus stimulation devices that are designed to restore hearing in patients with sensorineural deafness. These findings will also be of assistance to speech scientists and engineers designing real- time speech decoders. Since one of the major goals of this proposal is to increase knowledge of the functional organization of the auditory system, with particular emphasis on pathways that process features of complex sounds, the results will be particularly useful to neurologists and neurosurgeons in diagnosing the functional significance of lesions, cerebrovascular accidents and head trauma involving compromise of auditory function.